Cholesteric liquid crystal displays are characterized by the fact that the pictures stay on the display even if the driving voltage is disconnected. The bistability and multistability also ensure a completely flicker-free static display and have the possibility of infinite multiplexing to create giant displays and/or ultra-high resolution displays. In cholesteric liquid crystals, the molecules are oriented in helices with a periodicity characteristic of material. In the planar state, the axis of this helix is perpendicular to the display plane. Light with a wavelength matching the pitch of the helix is reflected and the display appears bright. If an AC-voltage is applied, the structure of the liquid crystals changes from planar to focal conic texture. The focal conic state is predominately characterized by its highly diffused light scattering appearance caused by a distribution of small, birefringence domains, at the boundary between those domains the refractive index is abruptly changed. This texture has no single optic axis. The focal conic texture is typically milky-white (i.e., white light scattering). Both planar texture and focal conic texture can coexist in the same panel or entity. This is a very important property for display applications, whereby the gray scale can be realized.
Current cholesterics displays are utilizing xe2x80x9cBragg reflectionxe2x80x9d, one of the intrinsic properties of cholesterics. In Bragg reflection, only a portion of the incident light with the same handedness of circular polarization and also within the specific wave band can reflect back to the viewer, which generates a monochrome display. The remaining spectrum of the incoming light, however, including the 50% opposite handedness circular polarized and out-off Bragg reflection wave band, will pass through the display and be absorbed by the black coating material on the back surface of the display to ensure the contrast ratio. The overall light utilization efficiency is rather low and it is not qualified in some applications, such as a billboard at normal ambient lighting condition. The Bragg type reflection gives an impression that monochrome display is one of the distinctive properties of the ChLCD.
In many applications, human eyes are friendlier with full color spectrum, i.e., white color information written on the dark background. With the development of the flat panel display, more and more displays with neutral color have come into being, such as black-and-white STN display and AMTFT display, etc. Unfortunately, both of these approaches involve major disadvantages and limitations. The AMTFT displays are not true zero field image storage systems because they require constant power input for image refreshing. The STN displays do not possess inherent gray scale capability as a result of the extreme steepness of the electro-optical response curve of the display. To realize a gray scale, the resolution has to be reduced by using, for example, four pixels instead of one per area. Anywhere from one to four pixels are activated at a particular time to provide the gray scale effect. The AMTFT devices use semiconductors to provide memory effects and involve use of expensive, ultra high resistance liquid crystal materials to minimize RC losses. Additionally, these displays are both difficult and costly to produce and they are, at present, limited to relatively small size displays. The cholesteric display has many advantages over the STN and AMTFT display with its zero field memory effect, hemispheric viewing angle, gray scale capability and other optical performances, but it obviously needs to come up with black-and-white solution in order to keep its superiority.
U.S. Pat. No. 5,796,454 introduces a black-and-white back-lit ChLC display. It includes controllable ChLC structure, the first circular polarizer laminating to the first substrate of the cell which has the same circular polarity as the liquid crystals, the second circular polarizer laminating to the second substrate of the cell which has a circular polarity opposite to the liquid crystals, and a light source. The black-and-white back-lit display is preferably illuminated by a light source that produces natural xe2x80x9cwhitexe2x80x9d light. Thus, when the display is illuminated by incident light, the circular polarizer transmits the 50% component of the incident light that is right-circularly polarized. When the ChLC is in an ON state, the light reflected by the ChLC is that portion of the incident light having wavelengths within the intrinsic spectral bandwidth, and the same handedness; The light that is transmitted through the ChLC is the complement of the intrinsic color of ChLC. The transmitted light has right-circular polarization, however, it is thus blocked by left-circular polarizer. Therefore, the observer will perceive that region of the display to be substantially black. When the display is in an OFF state, the light transmitted through the polarizer is optically scattered by the ChLC. The portion of the incident light that is forward-scattered is emitted from the controllable ChLC structure as depolarized light. The left-circularly polarized portion of the forward-scattered light is transmitted through the left-circular polarizer, thus, is perceived by an observer. The black-and-white display, in ""454 patent, is generated by back-lit component and the ambient light is nothing but noise.
U.S. Pat. No. 6,344,887 introduces a method of manufacturing a full spectrum reflective cholesteric display, herein is incorporated by reference. ""887 teaches a cholesteric display employing absorptive polarizers with the same polarity but different disposition. The display utilizes an absorptive circular polarizer and a metal reflector film positioned on the backside of the display to guide the second component of the incoming light back to the viewer. However, the shortcoming of the Iodine type absorptive polarizer makes the display to take on a tint of color in the optical ON state, for example, greenish white. The reasons for that are described as follows: Firstly, all the absorptive iodine polarizer has a more or less blue leaking problem which causes non-neutral color of a display device. Secondly, the absorptive polarizer has limited transmission (44%) and polarization efficiency that causes the second reflection having less intensity than that of the first one. Thirdly, the metal reflector always has a limited reflectivity. Take the Aluminum for example, the reflectivity is in the range of 80xcx9c90%. Fourthly, the quarter waveform retardation film can only match a narrow wavelength of the light to generate a circularly polarized light. Addition to the multi-layer surface mismatching, the total reflection of the back absorptive circular polarizer is around 35%. All those reasons result in a full spectrum cholesteric display appearing non-paper white.
It is the primary objective of the present invention to realize a paper-white reflection in display""s planar texture while maintaining the cholesterics"" superiority such as high contrast ratio, hemispheric viewing angle, zero-field long time memory and so on.
It is another objective of the present invention to create the dark state in display""s focal conic texture so as to achieve a reflective type black-and-white display.
It is still another objective of the present invention to realize a paper-white transmission in focal conic texture when the display switches to back-lit working mode.
It is also another objective of the present invention to create the dark state in planar texture when the display switches to back-lit working mode so as to achieve a transmissive type black-and-white display.
It is another objective of the present invention to accomplish a theoretically best brightness of the front reflection.
It is still another objective of the present invention to utilize a broadband reflective elliptical polarizer as a key component to generate and guide the elliptical polarization.
It is a further objective of the present invention to take the advantage of the bipolarity characteristics of the cholesteric broadband elliptical polarizer to achieve a high efficiency front-lit and back-lit converting system.
It is also another objective of the present invention to obtain a brightness enhancement effect of the reflective elliptical polarizer.